The Duron/Tbird is different from the Celeron/P3 combo in two ways. First, the Duron isn't a neutured Tbird like the Celeron is a neutured P3. Instead, the Duron is a similar, but different core. Intel litterally disables half the L2 cache to make a Celeron from a P3; but that costs the Celeron 128K of L2 cache, and brings the associativity down from 8way to 4way, which hurts performance more. The Duron, on the other hand, is just a Tbird not made with as much L2 cahce(64K instead of 256K). The associativity isn't cut at all, so it doesn't take the performance hit in that department.